The invention relates to a system for the active setting of a radial clearance size as well as to an aircraft engine.
In order to ensure efficient operation of an aircraft engine, it is expedient to keep the clearance between the tips of the rotating turbine blades or compressor blades and the surrounding housing as small as possible. Thus, what is known from GB 2417762, for example, is a method for active clearance control (tip clearance control, TCC). Here, the housing is cooled in a settable manner by means of cooling air, namely in such a way that a defined clearance appears between the rotating rotor blade tips and the housing (or the cover bands or liners which are arranged thereat).
At that, the setting, i.e. the regulation or control of the cooling, is taken over by the engine computer (EEC) that selects a corresponding valve. The goal here is to set a clearance that is as small as possible, while at the same time any wear due to grazing, i.e. any running of the blade tip into the housing or the cover band, has to be avoided.
Systems that are modulatable in a fully modular manner are operated by means of a method in which the actual clearance is either measured or calculated by the engine computer. The actual clearance that is thus determined is compared to the target clearance, and the housing cooling is adjusted correspondingly by the control system.
Usually, the target clearance is set to be zero when the aircraft engine provides maximum thrust. The value for the target clearance can be higher than zero when the aircraft engine runs in the partial-load range. In this case, a sudden increase in thrust leads to a closure of the clearance, which cannot be compensated for fast enough by the reduced housing cooling, that is, it cannot keep the clearance constant. In this case, a target clearance that is too small would lead to a grazing contact.